1. Field of the Invention
This invention relates to electronic parts, such as an inductor, used for eliminating electromagnetic interference generated within signal lines of electronic equipment, and a method for making same.
2. Description of the Related Art
A number of such electronic parts for noise rejection and their methods for making have been proposed in the art as in the following.
A typical example of conventional methods is that a mixture of ferrite and a binder is charged to a mold in which a metal line, for example, a platinum line, has previously been arranged, followed by compression molding, fitting of external electrodes and sintering to form ferrite bead inductors provided with a through-hole type electrode therein (conventional example 1).
In an another example, a kneading mixture of ferrite powder and a resin binder is charged to an extrusion mold in which a metal line has been arranged, followed by extrusion molding of the thus kneaded mixture with the metal line pierced therein from an extruder head, cutting to a desired length, fitting external electrodes and sintering to form electronic parts (conventional example 2). A conventionally produced electronic part is shown in
Such electronic parts further include those disclosed in a Japanese Patent Kokai No. 63-226,904 (conventional example 3) as an example of a tip inductor in which a conductive metal wire is embedded into a ferrite material. In this inductor, a magnetic layer is monolithically adhered to a periphery of a conductive material by extrusion molding to form a molded part in continuous length as an inductance element.
Although such electronic parts can be produced by conventional methods, there are still problems to be solved, as will be described below.
The first problem is caused by the metal line. According to conventional examples, the molded material comprising the metal line (L') embedded therein is cut into a desired length. As the metal line (L') is fine, it tends to fall off from the ferrite molded material due to vibration or shock during the subsequent process to fit terminals.
The second problem is caused by the sintering treatment. When the mixture of ferrite and a binder is sintered at a temperature of, for example, 800.degree. C. to 1200.degree. C., the ferrite material shrinks itself about 15% and, on the contrary, the metal line in the molded material expands several percent. As a result, the metal line breaks the external electrodes (T') fitted to both sides of the material which allows both cut ends of the line to protrude outside as shown in FIG. 7(A). Such a protrusion of the line hinders a uniform deposition of the external electrodes on the material during the dipping treatment. Especially, if the line (L') protrudes exclusively from one side of the ferrite core as shown in FIG. 7(B), the dimensional size of the external electrodes varies widely, which would cause failure of soldering, such as so-called Manhattan phenomenon.
The third problem is caused by an incomplete contact between the external electrodes and the conductive material. As an area used for their contact is small and is thus called a "point contact", each electronic part would be deflected to disconnect between the electrodes and the conductive material when a circuit substrate consisted of such parts is bent intentionally.
Further, in the conventional example 3 described above, the magnetic layer is adhered throughout the outer periphery of the conductive material by extrusion molding, which results in internal stress between these two materials due to a difference in shrinkage factor thereof during a sintering treatment of the magnetic material. This should result in poorer adhesive properties between two materials and a decrease in, for example, the intrinsic impedance properties, which leads to deterioration of products although usefulness of the inductance element is not remarkably spoiled.
Conventional problems as described above can be solved and electronic parts such as an inductor in higher quality are provided by the present invention.